The present invention relates to fluoropolymer aqueous dispersions to be used in coating applications for metal and ceramic surfaces and in the textile impregnation.
Specifically, the present invention relates to films obtained from fluoropolymer aqueous dispersions, having an high critical thickness, preferably combined with good optical and mechanical properties.
Critical thickness means the maximum thickness obtainable without cracks and surface defects inside the film.
It is very important from the application point of view to increase the limit of the critical thickness so as to have films without cracks in correspondence with the highest thickness. An higher critical thickness means indeed better film mechanical (e.g. scratch-resistance) and optical properties.
It is known in the art to combine two latexes so as to have a fluoropolymer aqueous dispersion having a bimodal distribution of the particle diameters to prepare useful dispersions in coating applications on metal, glass and ceramics or glass-fiber textiles. Specifically see EP 657,514 which describes the use of fluoropolymer dispersion mixtures, expressly excluding thermoplastic polymers, obtained by emulsion polymerization and mixed to obtain bimodal distributions of the particle sizes in order to optimize, in particular in the textile impregnation sector, the applied polymer amount for each passage without crack formation. The ratio between the particle sizes having a lower diameter with the particle size having an higher diameter is in the range 0.3-0.7. The first dispersion has an average diameter by number in the range 180-440 nm, while the second fluoropolymer dispersion has an average diameter from 50 to 150 nm. The component amount having a lower diameter with respect to the component having an higher diameter is in the range 5-50% by weight, preferably 5-20% by weight. The Examples reported in this patent substantially relate to the textile impregnation and show that, by using an amount of 10% and 18% by weight of the component with the lower size, cracks are eliminated and the amount of the applied fluoropolymer increases. If amounts other than these two values are used, cracks are noticed. The polymer having a lower size has particle sizes of 100 and 110 nm and the above mentioned ratio ranges from 0.45 to 0.5. The only example given on the metal coating shows that with an amount equal to 10% of the fluoropolymer having lower particle size, the film hardness increases, in comparison with the case where the film is obtained by exclusively using the flouropolymer a) with higher sizes as above defined. Tests carried out by the Applicant have shown that in the case of metal coating, by using the dispersions exemplified in the above mentioned patent, there is no increase of critical thickness.
The need was felt to have available fluoropolymer aqueous dispersions able to form films on metals having an higher critical thickness combined with the absence of cracks, preferably with improved optical properties, with respect to the films obtained from the fluoropolymer dispersions of the prior art.
The Applicant has surprisingly found that by using a mixture of two fluoropolymer dispersions having a different granulometric distribution and such that the ratio between the average sizes of the respective particles is well defined, films having the above mentioned properties are obtained.
An object of the present invention are therefore fluoropolymer dispersion mixtures, wherein:
a) a dispersion is formed by tetrafluoroethylene (TFE) homopolymers or by its copolymers with one or more monomers containing at least one ethylene type unsaturation in amounts from 0 up to 8% by weight, preferably from 0.01 to 3% by weight; the average particle sizes range from 190 to 400 nm, preferably from 210 to 300 nm;
b) another dispersion is selected from one or more of the following dispersions:
1) homopolymer dispersion of tetrafluoroethylene (TFE), or based on its copolymers with one or more monomers containing at least one ethylene type unsaturation in amounts from 0 up to 8% by weight, preferably from 0.01 to 3% by weight;
2) TFE thermoplastic copolymers, preferably copolymers containing from 7 to 27% by weight of hexafluoropropene; copolymers containing from 0.5 to 18% by weight, in particular from 2 to 10% by weight of one or more perfluoroalkylvinylethers, preferably selected from methyl-, ethyl-, propylvinylether;
the average particle sizes of the dispersion b) range from 20 to 60 nm, preferably from 20 to 45;
the ratio between the average particle sizes of the dispersion b) with respect to those of the dispersion a) being lower than 0.3, preferably 0.1 to 0.25, component b) being obtainable by emulsion polymerization.
The dispersion b) can be used even though the average particle size is higher than 60 nm, provided that the particle fraction having sizes defined in b) (average particle size 20-60 nm) is equal to at least 60% by weight, preferably 70% by weight, the ratio by weight between the average particle sizes of 20-60 nm and those of dispersion a) being lower than 0.3.
The Applicant has found that in order to obtain the present invention results, it is important to have at least two dispersions having the above mentioned granulometric distributions. If desired, one or more dispersions having distribution with an average diameter lower than 20 nm, can be added.
Among the TFE comonomers the fluorinated ones are specifically mentioned:
C3-C8 perfluoroolefins, such as hexafluoropropene (HFP);
C2-C8 hydrogenated fluoroolefins, such as vinyl fluoride (VF), vinilydene fluoride (VDF), trifluoroethylene, hexafluoroisobutene, CH2xe2x95x90CHxe2x80x94Rf perfluoroalkylethylene, wherein Rf is a C1-C6 perfluoroalkyl;
chloro- and/or bromo- and/or iodo- C2-C8 fluoroolefins, such as chorotrifluoroethylene (CTFE);
CF2xe2x95x90CFORf (per)fluoroalkylvinylethers (PAVE), wherein Rf is a C1-C6 (per)fluoroalkyl, for example CF3, C2F5, C3F7;
CF2xe2x95x90CFOX (per)fluoro-oxyalkylvinylethers, wherein X is: a C1-C12 alkyl, or a C1-C12 oxyalkyl, or a C1-C12 (per)fluoro-oxyalkyl having one or more ether groups, for example perfluoro-2-propoxy-propyl.
The comonomers which do not substantially decrease the PTFE thermal stability, are preferred.
The preferred fluoropolymers for the dispersion a) are polytetrafluoethylene or polytetrafluoroethylene modified with one or more comonomers having ethylene unsaturation.
The preferred fluoropolymers for the dispersion b) are polytetrafluoroethylene or modified polytetrafluoroethylene. When an improvement of the optical and mechanical properties is desired, thermoplastic fluoropolymers of the type b2), preferably TFE copolymers with perfluoromethylvinylether in amounts between 6 and 7% by weight, optionally containing perfluoropropylvinylether between 0.8 and 1.2% by weight, or TFE copolymers with perfluoropropylvinylether from 4 to 6% by weight, are used.
The type a) aqueous dispersions are obtainable by the conventional emulsion polymerization processes.
The type b) aqueous dispersions are obtainable with the polymerization process in microemulsion described in the Italian patent application MI 98A001519 in the name of the Applicant, filed on the same day as the present application and having for title xe2x80x9cTFE polymerization processxe2x80x9d. The process relates to the preparation of dispersions based on tetrafluoroethylene (TFE) homopolymers, or based on its copolymers with one or more monomers containing at least one ethylene type unsaturation, having a particle fraction equal to at least 60% by weight, preferably 70% by weight, with average diameter sizes in the range 0.005-0.06 xcexcm, preferably 0.01-0.05 xcexcm, comprising:
a) preparation of an aqueous microemulsion of perfluoropolyethers (PFPE) having neutral end groups or end groups optionally containing 1 or more H atoms, Cl instead of fluorine;
b) feeding of the microemulsion to the polymerization reactor, in such amount wherefore the microemulsion perfluoropolyether oil phase is present in a concentration higher than 2 ml per liter of reaction medium, preferably from 2.2 ml up to 50 ml per liter, still more preferably from 3 to 30 ml per liter of reaction medium;
c) feeding of the reaction medium into the polymerization reactor, reactor degassing, reactor pressurization with gaseous TFE, optional addition of surfactants, stabilizers, comonomers, chain transfer agents;
d) initiator addition, and optionally during the polymerization of further amounts of surfactants, stabilizers, comonomers, chain transfer agents;
e) discharge from the reactor of the polymeric latex.
In order to obtain the results of the present invention the weight ratio between the component a) and component b) as dry product can range between 99/1 and 90/10, preferably between 99/1 and 95/5 by weight.
The dispersions are usually used at a concentration of the dry product in the range 25%-75% by weight and preferably 40-65% by weight.
The mixture can be obtained by simple mixing the component a) previously concentrated by the known methods (addition of non ionic surfactant and heating or ultrafiltration) with the component b) so as it is obtained from the polymerization autoclave or concentrated as above for component a) or it can be obtained by coconcentration of the two latexes.
The component b) can be concentrated as it is obtained from the polymerization autoclave or after ultracentrifugation. In this case the surfactant dispersion of the ultracentrifugation is used.
In particular by using mixtures of dispersions constituted by the component a) and by the component b), preferably in the composition range from 99/1 to 95/5 by weight, and with the above mentioned particle size ratio, it is possible to considerably increase the critical thickness of the films obtained starting from these mixtures in comparison with the films obtained from the sole component a).
As already said, when in the invention mixtures a thermoprocessable polymer is used as dispersion b), gloss and scratch-resistance of the obtained films are improved.
The obtained dispersion mixture can be suitably formulated in connection with the intended application, with the addition of other resin aqueous dispersions such as for example acrylic resins, silicone resins, polyamidoamidic resins, sins, etc.; pigments, surfactants, inorganic fillers and other additives. After the mixture application to the desired surface, the film is dried and then sintered at a temperature higher than t he polymer melting temperature.
The total surfactant amount necessary to stabilize the mixture of the invention dispersions generally ranges between 2 and 10% and it is preferably between 3 and 6% by weight of the dispersion.
The fluoropolymer aqueous dispersions of the present invention besides for coating applications on metal surfaces can be used also for ceramic surfaces and in the textile impregnation and for obtaining cast films.